x11

X11 is the 11th release of the X Window System; it is currently at
revision 6.6 (i.e. X11R6). It's the name for the protocol used in
current X implementations. The standard was ratified in 1986 - the same
year this presenter was born (!).

X11 is implemented by absolutely everyone, including
XFree86, Xi, Solaris, and every commercial UNIX in the
past few years.

X11 is flexible thanks to its extensions
architecture.

X11 is window-based (even the background you see is
a window - the "root window"), and event-based (clicks,
mouse-in, mouse-out, etc).

X11 is fully network-transparent: applications can
operate on either network or local connections, not
needing to have to know which.